BEST MANAGEMENT PRACTICES MANUAL

1 best MANAGEMENT PRACTICES MANUAL Published by Environmental Section of the Idaho Transportation Department August 2011 best MANAGEMENT PRACTICES MANUAL 08/11 Chapter 5 Post-Construction CHAPTER 5 POST-CONSTRUCTION Introduction Permanent erosion and sediment control best MANAGEMENT PRACTICES (BMPs) are long-term measures that survive the design life of a project. The area where permanent BMPs have been applied should remain permanently stabilized until the area is disturbed again due to construction or in the case of maintenance, slides or floods. Permanent BMPs are designed to reduce or control erosion and are put in place during construction with beneficial results extending over a period of years.

2 Permanent erosion control is the prime consideration in a soil-disturbing project and, in contrast to temporary erosion control, is designed and planned for long-term benefits. Post-Construction MANAGEMENT Goals Permanent erosion and sediment control goals consist of: 1. Perimeter Controls a. Ensure that no sediment is leaving the project area. 2. Controls within the Project a. Use erosion and sediment controls that perform properly. b. Minimize or eliminate erosion. c. Complete stabilization of the site. 3. Final Product a. Complete stabilization and no erosion prior to final acceptance. b. Achieve Notice of Termination. best MANAGEMENT PRACTICES (BMP) Post-construction involves the use of the following BMPs: PC-1 Channel Protection Check Dams PC-2 Sheet Flow to Buffers PC-3 Channel Protection Flexible Liners PC-4 Channel Protection Rigid Channel Liner PC-5 Dikes and Berms PC-6 Dry Swale PC-7 Wet Swale PC-8 Geosynthetics PC-9 Surface Sand Filter PC-10 Subsurface Sand Filter PC-11 Perimeter Sand Filter PC-12 Organic Filter PC-13 Pocket Sand Filter best MANAGEMENT PRACTICES MANUAL 08/11 Chapter 5 Post-Construction PC-14 Bioretention PC-15 Inlet/Outlet Protection PC-16 Interceptor Ditches PC-17 Retaining Walls PC-18 Stormwater Basins PC-19 Extended Detention Basin with Micropool PC-20 Wet Basin PC-21 Wet Extended Detention Basin PC-22 Shallow Wetland PC-23 Extended Detention Shallow

3 Wetland PC-24 Pond/Wetland System PC-25 Pocket Wetland PC-26 Sediment Control Box PC-27 Infiltration Trench PC-28 Infiltration Basin PC-29 Slope Drains/Chutes/Flumes PC-30 Rock Armor/Rock Mulch/Turf Reinforcement Mats PC-31 Serrations and Roughening PC-32 Terraces and Benches PC-33 Top Soil MANAGEMENT PC-34 Vegetation/Seeding PC-35 Vegetation/Planting PC-36 Water Quality Inlet Oil/Grit Separator PC-37 Street Sweeping PC-38 Deep Sump Catch Basin PC-39 On-Line Storage in Storm Drain Network (Vaults) PC-40 Porous Pavements PC-41 Proprietary/Manufactured Systems best MANAGEMENT PRACTICES MANUAL PC-1 Channel Protection Check Dams 08/11 Chapter 5 Post-Construction PC-1 CHANNEL PROTECTION CHECK DAMS Refer to: ITD Standard Specifications, Section 212.

4 ITD Standard Drawing P-2-B. Definition and Purpose A check dam is a small dam constructed in an open channel, swale, or drain way to reduce or prevent excessive bank and bottom erosion by reducing the gradient or runoff velocity. Check dams are normally made of rock; however, other materials may be used. (Rock check dams are often used with channel liners to prevent formation of rills and gullies, or to interrupt their growth, and/or to provide grade control in the channel.) Bank barbs are similar to rock check dams in construction and materials. Bank barbs are a measure to guide the force of the stream or channel flow away from the bank to reduce bank erosion. Bank barbs may be required under an Idaho Department of Water Resources Stream Alteration Permit, are beyond the scope of this MANUAL , and require guidance from a Professional Engineer.

5 Appropriate Applications Check dams are often used in channels or ditches where adequate vegetation cannot be established for controlling erosion and may be used below small drainage structures or culverts. Limitations Check dams are not to be used in live streams, except with proper permitting in place (Idaho Department of Water Resources, Army Corps of Engineers). Use of check dams below the high water mark of a stream or other water body (waters of the ) should be carefully evaluated due to Section 404 permit requirements. Section 404 permitting and an Idaho Department of Water Resources Stream Alteration Permit may be required. These are required for the use of bank barbs in live streams. BMP Objectives Perimeter Control Slope Protection Borrow and Stockpiles Drainage Areas Sediment Trapping Stream Protection Temporary Stabilizing Permanent Stabilizing best MANAGEMENT PRACTICES MANUAL PC-1 Channel Protection Check Dams 08/11 Chapter 5 Post-Construction Design Parameters Maximum height should be 24 inches.

6 The center of the check dam must be 6 inches lower than either edge, to form a weir at the overflow. The sides should also be lower than the adjoining banks, roadway, or backslope. The top of the outside edges should be 6 inches lower than the roadway surface to prevent water from flowing onto the roadway or undercutting the banks. The drainage area above the check dam should not exceed 10 acres. The dams must be spaced so that the toe of the upstream dam is never lower than the overflow of the downstream dam. Excavating a sediment basin immediately upstream from the check dam improves its effectiveness. Check dams composed of rock may be placed on erosion control geotextile to avoid undercutting. Be sure to check that the overflow will handle large volumes of water and that the sides are high enough to form a weir.

7 Within the safety clear zone, all rock check dam face slopes shall be 6H:1V or flatter relative to the roadway grade. Overflow channel slopes may be 3H:1V or flatter. Outlet stabilization should be provided below each check dam, and the use of channel liners or protection such as riprap should be considered where there is potential for significant erosion or prolonged submergence. Materials Rock size should vary from 1 to 8 inches with the 8-inch material making up approximately 30 percent of the mix. Erosion control geotextile shall meet the requirements of the Standard Specifications. Construction Guidelines The rock check dams shall be constructed according to the plans and specifications. The rock shall be placed on erosion control geotextile either by hand or using appropriate equipment.

8 Rock shall not be dumped directly on the geotextile. The upstream side of the dam shall be lined with a layer of to 2-inch Coarse Aggregate for concrete 6 inches deep if necessary for additional channel protection. Riprap and erosion control geotextile may be necessary on the downstream side of the dam to protect the streambed channel from scour. Rock check dams within the safety clear zone shall have a slope of 6H:1V or flatter relative to the roadway grade. Maintenance and Inspection Conduct inspections as required by the NPDES permit or contract specifications during construction. Periodic inspection and maintenance will be required based on post-construction site conditions. best MANAGEMENT PRACTICES MANUAL PC-1 Channel Protection Check Dams 08/11 Chapter 5 Post-Construction Make any repairs necessary to ensure the measure is operating properly.

9 Remove accumulated debris and sediment from behind the dam when the debris or sediment reaches a depth of one-half the original height of the dam. Properly dispose in an approved location. Restore rock as necessary to maintain the correct dam height. best MANAGEMENT PRACTICES MANUAL PC-2 Sheet Flow to Buffers 08/11 Chapter 5 Post-Construction PC-2 SHEET FLOW TO BUFFERS Reference: Denver Urban Drainage and Flood Control District, Volume 3 Criteria MANUAL . Definition and Purpose This is a structural BMP used to filter pollutants as stormwater runoff moves to a swale, stream, or other flow area. It protects streams, lakes, and/or wetlands from high concentrations of sediment in runoff. The flows are discharged over the buffer zone where sediments and other pollutants can be filtered out before the flows reach the natural drainage way.

10 Appropriate Applications Zones where stormwater runoff is treated by a natural buffer before it enters a stream or forested area. Runoff from pervious and impervious areas is discharged through buffer. Buffer generally consists of grass, meadow, forest, or a mix. Generally used to treat overland flow in the green space of a development site. Level spreader or similar BMP can be used along upstream edge of buffer zone to enhance treatment. Design Parameters Minimum buffer width is 50 feet and is measured from the bank elevation of the stream. Maximum contributing length is 150 feet for pervious surfaces and 75 feet for impervious surfaces. Runoff will enter the buffer as sheet flow. If sheet flow cannot be achieved at the edge of the buffer, a level spreader or similar BMP will be used to establish sheet flow.